1. Field of the Invention
The present invention relates to a method for producing a bulk single crystal of silicon carbide (SiC) of good crystalline quality which is suitable for use as a substrate material for optical devices and electronic devices, and more particulary to a method capable of stably producing such a silicon carbide single crystal at a temperature of 2000° C. or lower, which is preferred from the viewpoint of operation. The present invention also relates to a bulk silicon carbide single crystal of good crystalline quality which is thus prepared.
2. Description of Related Art
Silicon carbide (SiC) is a class of thermally and chemically stable compound semiconductors and is characterized by having a band gap of about three times, a dielectric breakdown voltage of about ten times, a rate of electron saturation of about two times, and a coefficient of thermal conductivity of about 3 times as large as that of silicon (Si). In view of these favorable properties, silicon carbide is expected to be useful in applications as a substrate material for power devices which surpass the physical limitations of Si devices and for environment-resisting devices which operate at high temperatures.
In optical devices, materials based on gallium nitride (GaN) are being developed for shortening the operating wavelength. Silicon carbide has a significantly smaller lattice mismatch with GaN compared to other compound semiconductors, so it attracts attention as a substrate material for use in epitaxial growth of a GaN layer.
It is well known that silicon carbide occurs in polytypes which differ from each other with respect to the stacking sequence of atomic layers (SiC biatom layers) in the c-axis direction. The most common polytypes are 3C, 6H, 4H, and 15R wherein the suffixes C, H, and R indicate the cubic, hexagonal, and rhombohedral structures, respectively, and the number preceding each suffix indicates the number of biatom layers stacked within each cycle of the crystals. Thus, 3C means a cubic crystal having a cycle in which three biatom layers are stacked; 4H and 6H mean hexagonal crystals having a cycle in which four and six biatom layers, respectively, are stacked; and 15R means a rhombohedral crystal having a cycle in which 15 biatom layers are stacked.
In order to apply silicon carbide to electronic or optical devices, a bulk silicon carbide single crystal of good crystalline quality having no or quite few defects is required. The below-mentioned physical vapor transport (PVT) technique (which is the technique commonly employed in the production of SiC single crystals) tends to result in incorporation of other crystal forms due to the occurrence of polytypical transformation during growth of an SiC single crystal. This leads to the occurrence of structural defects such as micropipes and stacking faults. Micropipes are defects in the form of hollow cores having a diameter of about 2 micrometers or greater, resulting from a large Burgers vector for dislocation in crystal, leaving the dislocation line as a hollow defect. Stacking faults are caused by disorder or disturbance in stacking sequence of layers due to dislocation in crystal. In particular, micropipes are fatal defects, and those portions of a single crystal including micropipe defects cannot be used for substrates.
The methods for the production of silicon carbide which are known in the prior art include a physical vapor transport (PVT) technique and a chemical vapor deposition (CVD) technique, both belonging to a vapor phase growth method, the Acheson method, and a solution growth technique which belongs to a liquid phase growth method (also called a liquid phase epitaxy [LPE] method).
The PVT technique comprises subliming a silicon carbide powder used as a raw material at a high temperature in the range of 2200-2500° C. and depositing silicon carbide on a seed (crystal) substrate of a silicon carbide single crystal which is placed in a region having a lower temperature. The CVD technique uses a silane gas and a hydrocarbon gas as raw materials for a vapor phase reaction, thereby causing epitaxial growth of a silicon carbide single crystal on a heated substrate of a material such as Si.
The Acheson method has been utilized for a long time in commercial production of silicon carbide crystals for use as an artificial abrasive. It is a method in which silicon carbide is prepared by loading anhydrous silicic acid and carbon around a carbon electrode and heating the loaded materials to a high temperature of 2500-2700° C. by passing electricity through the electrode. Single crystals of silicon carbide are produced as by-products.
The solution growth technique comprises melting Si or an Si-containing alloy in a graphite crucible, thereby causing carbon to dissolve from the crucible into the resulting melt to form a molten solution, and allowing a layer of silicon carbide single crystal to grow on a seed substrate placed in a lower temperature region of the solution by deposition from the liquid phase (molten silicon carbide solution). In a variation of the solution growth technique using an Si-containing alloy, a solution is prepared by dissolving C and Si in molten Cr, and a silicon carbide single crystal is allowed to grow from the molten solution in the same manner as above.
Silicon carbide single crystals grown by the PVT technique include many structural defects such as micropipe defects and stacking faults. In the PVT technique, SiC vapor is not present in the sublimated gas. Instead, Si, Si2C, and SiC2 vapors which are vaporized from a silicon carbide powder and C vapor which is vaporized from a graphite jig or tool coexist in the sublimated gas. The formation of many structural defects in the PVT technique is caused by the facts that it is quite difficult to control the partial pressures of these various vapors in a stoichiometric manner and that complicated reactions are involved in the crystal growth.
Nevertheless, bulk silicon carbide single crystals have mostly been produced by the PVT technique for the reason that it is extremely difficult for the other techniques to stably produce a bulk silicon carbide single crystal at a satisfactory growth rate, as described below. Because bulk silicon carbide single crystals produced by the PVT technique include many micropipe defects, it is difficult to produce from these crystals semiconductor devices of a square shape measuring a few millimeters with a good yield. Researches on the PVT technique with the aim of reducing the occurrence of micropipe defects has been done actively, but there have been no reports indicating that a micropipe-free bulk silicon carbide single crystal which contains substantially no micropipe defects has been obtained by the PVT technique.
The CVD technique is not suitable for the production of a bulk single crystal, which is required to manufacture a silicon carbide substrate, due to limited amounts of the raw materials being supplied in gaseous states. Therefore, the CVD technique is utilized solely for the growth of a silicon carbide crystal in the form of a thin film.
It is not possible for the Acheson method to produce a pure silicon carbide single crystal due to contamination of the raw materials with many impurities, and it is also not possible for that method to obtain a single crystal having a size sufficient for use as a substrate material.
In the solution growth technique, crystal growth proceeds in conditions close to thermal equilibrium, and thus a silicon carbide single crystal of good crystalline quality having an extremely low number of structural defects is obtained. However, since the concentration of carbon dissolved from a graphite crucible into a melt is low, the growth rate of a silicon carbide crystal is very slow. In the case of an Si solvent method in which the solvent material to be melted is Si, the growth rate of a silicon carbide crystal is said to be 5-12 μm/hr when the temperature of the melt is 1650° C. This growth rate is one or two orders of magnitude lower than that of the PVT technique. It is theoretically possible to increase the growth rate by elevating the temperature of the melt to 2000° C. or higher so as to increase the concentration of carbon which can be dissolved into the melt, but this results in severe evaporation of the Si solvent at atmospheric pressure, or if the melt is pressurized, the apparatus becomes complicated, either result being problematic for commercial production.
It has been attempted to add a transition metal or a rare earth metal such as Cr or Sc to a molten solvent of Si in order to increase the growth rate of a silicon carbide crystal by increasing the concentration of carbon dissolved in the molten solvent while suppressing the evaporation of the molten solvent. However, even if such a technique is employed, a product which has been actually obtained by the Si solvent method is merely a thin film having a thickness on the order of a few micrometers. Accordingly, like the CVD technique, the solution growth technique has been considered to be a technique suitable for forming a thin film on a seed substrate but unsuitable for the growth of a bulk single crystal.
JP-A 2000-264790 discloses that a raw material comprising at least one transition metal, Si, and C is heated to melt in a carbonaceous crucible (which is actually a graphite crucible) to form a molten solution, and a bulk silicon carbide single crystal is deposited and grown on a seed substrate by cooling the molten solution or forming a temperature gradient in the molten solution. It is described therein with respect to a system of 31Mo-66Si-3C, 54Cr-23Si-23C, or 29Co-65Si-6C, for example, that the growth rate at a temperature of 1750-2150° C. was 200-800 μm/hr on the average, although it depended on the temperature gradient in the molten solution.
In that method, however, carbon is not supplied by dissolution from the graphite crucible. Instead, the crucible is charged with a raw material containing a predetermined amount of carbon. Therefore, competitive dissolution of carbon from the crucible and from the raw material occurs inevitably. As a result, part of the carbon present in the raw material may remain undissolved in the molten solution, and the undissolved carbon may act as nuclei on which silicon carbide precipitates, thereby interfering with crystal growth on a seed substrate immersed in the molten solution and making it easy for subsequently growing silicon carbide crystals to become polycrystalline. The present inventors ascertained that a growth rate of only at most 100 μm/hr could be obtained at a temperature of 2000° C. or below.